Installation problems Installation problems – My IT experience

Centos Docker container su: cannot open session: Permission denied

Issue: After an software update in a Centos 7 docker container `su – user` is no longer possible with the following error

bash-4.2# su - username
Last login: Wed Sep 13 13:20:31 UTC 2017
su: cannot open session: Permission denied

Inappropriate settings of nofile in either in




There are several solution, which suggest removing nofile unlimited like editing limits.conf and Redhat proposed solution.

However there are also files under /etc/security/limits.d/, where you need to fix nofile references as well. Where you need to change it from unlimited or a number like 500000 to 65536 or less.

bash-4.2# cat /etc/security/limits.d/50-open-files.conf
*         hard    nofile      500000
*         soft    nofile      500000

Need to be edited to become:

bash-4.2# cat /etc/security/limits.d/50-open-files.conf
*         hard    nofile      65536
*         soft    nofile      65536

Docker inspect list ports, volumes and etc.

The `docker inspect` returns useful information about Docker containers.

To filter the returned input one can request a return format.

Main documentation can be found here:

A peace of information that is not immediately visible, but is always of interest is the list of volumes bonded, which can be extracted as follows:

docker inspect --format='{{json .HostConfig.Binds}}' container_name

To retrieve a list of the binds separated by a new line:

docker inspect --format='{{json .HostConfig.Binds}}' container_name | \
sed 's/"//g;s/\[//g;s/\]//g'| \
tr ',' '\n' 

For the network settings:

docker inspect --format='{{json .NetworkSettings.Ports}}' container_name

To find the command which was used when the docker was started:

docker inspect  -f "{{.Name}} {{.Config.Cmd}}" container_name

To find the environment variables with which the container was started:

docker inspect --format "{{.Config.Env}}" container_name

Many more useful inspect format magics could be found here:

Docker Inspect Template Magic

HTCondor Docker universe throws core.STARTER

This is a problem observed when using HTCondor in the Docker universe.

After re configuring HTCondor and Docker on one processing node, every time a job is sent the following errors are dumped in the corresponding slot’s StarterLog.slot1_N:

(pid:24877) Found 33 entries in docker image cache.
Stack dump for process 24877 at timestamp 1497439919 (13 frames)

There is also a core.STARTER generated and the output of `gdb /var/log/condor/core.STARTER <<< “where”` is:

Core was generated by `condor_starter -f -a slot1_1’.

(gdb) Python Exception <class ‘gdb.MemoryError’> Cannot access memory at address 0xb1340bc0:

The lead to that was a bug in the Docker thinpool storage driver, which led to the use of overlay2 driver alongside with a Docker reinstall.

There are ‘hidden’ dot files in the condor log directory, they contain cache information that might mess up with you job submission, to fix that one needs to stop condor, remove those files and start condor again. Once done the node start accepting Docker Universe jobs again.

systemctl stop condor
cp /var/log/condor/.s* /tmp/
rm -f /var/log/condor/.s*
systemctl start condor

Debian, The following packages cannot be authenticated!

WARNING: The following packages cannot be authenticated!

This is a Debian warning after an:

sudo apt-get update
sudo apt-get upgrade

After going through many sources around the correct solution turned out to be be:

sudo rm -r /var/lib/apt/lists
sudo mkdir -p /var/lib/apt/lists/partial
sudo apt-get update

Now you should be able to run the upgrade.


How to boost your wifi signal?!

You have an Internet connection that has been connected to a wireless emitter in order to provide wireless connectivity, but in some areas you have a lack of signal. What can you do to strengthenĀ  that wifi signal up?

In this article I will try to give you some simple advices to help you improve your wireless device performance.

Let’s assume that you do not have the opportunity to move your device along, otherwise this is one of the best ways to adjust your signal. So here is what you can do:

1. Adjust wireless device antenna direction. The best direction of your antenna is the one that has a 90 degree angle (see the picture bellow) with the imaginary line drawn between the wireless device and your laptop (or any other wireless device, the green line between the laptop and the wireless).


On the figure shown above, the best direction of your antenna is shown in green. The blue position of the antenna shows a relevantly good antenna position. The position coloured in red is worse in this particular case, the weakest signal follows the direction of the antenna (0/180 degrees with the imaginary line of the position of your laptop). If the antenna of your wireless emitter point toward your laptop – you will experience a low signal!

2. Sometimes in the security settings of your wireless emitter, there is an option that lowers the power of the wifi signal for security reasons. Check if your device has such options, make sure to allow full signal strength.

3. If the first two steps are not applicable for any reason, you can boost your signal via wifi antenna booster such antenna can increase the level of you signal. The price of such boosters is near 5 US dollars 3.5 Euros. It attaches to your device just as a regular antenna and it’s fairly easy for setting up.

For other solutions, on larger areas, that require higher levels of the signal an wifi signal amplifier might be a good choice. Such amplifiers might require separate power supply.